1. Field of the Invention
The present invention relates to a so-called composite thermal transfer sheet constructed of a thermal transfer image-receiving sheet having a receptor layer on a substrate and a thermal transfer film having a heat-fusible ink layer on a base film, peelably bonded to each other, such that the receptor layer of the thermal transfer image-receiving sheet and the heat-fusible ink layer of the thermal transfer film are peelably attached to each other.
The present invention also relates to a thermal transfer image-receiving sheet having a receptor layer on a substrate to be used for the composite thermal transfer sheet.
2. Description of the Related Art
Recently, a thermal transfer medium of a heat-fusible transfer type is often used for output prints of a computer, a processor or the like.
As one example of such a thermal transfer medium of the heat-fusible transfer type, a thermal transfer film of the heat-fusible transfer type is constructed of: a base film composed of a plastic film having a thickness of about 3.mu.m! to 20.mu.m! such as a polyester film or a cellophane film; and a heat-fusible ink layer, which is coated on the base film and which is composed of a mixture of vehicle (e.g. wax) and colorant such as pigment or dye.
When printing is affected on a thermal transfer image-receiving sheet using the aforementioned thermal transfer film, the thermal transfer film is supplied from a roll thereof, while a continuous or cut-sheet-like thermal transfer image-receiving sheet is also supplied, so that the former and the latter are superposed on each other on a platen. Then, in such a state, heat is applied to the thermal transfer film from the back side surface thereof by a thermal head, so that the heat-fusible ink layer of the thermal transfer film is melted and transferred to the thermal transfer image-receiving sheet. Therefore, a desired image is formed on the thermal transfer image-receiving sheet.
However, if the aforementioned thermal transfer film would be applied to a thermal printer used for printing on a heat-sensitive color-forming paper, a problem is posed as followings. Namely, the heat-sensitive color-forming paper develops coloring by itself (i.e. without the thermal transfer film). Therefore, the thermal printer used for printing on a heat-sensitive color-forming paper does not have any means for transporting the thermal transfer film. Thus, the thermal transfer film cannot be used in the aforementioned thermal printer.
In order to solve the aforementioned problem, the composite thermal transfer sheet is proposed. The composite thermal transfer sheet is constructed by peelably bonding a thermal transfer film and a thermal transfer image-receiving sheet. The thermal transfer film is constructed of a base film and a heat-fusible ink layer formed thereon, and a thermal transfer image-receiving sheet is constructed of, for example, a plain paper, a synthetic paper, a coated paper and so on. Further, the thermal transfer film and the thermal transfer image-receiving sheet are peelably bonded through a temporary adhesive layer formed on the heat-fusible ink layer of the thermal transfer film. Furthermore, in the composite thermal transfer sheet, the thermal transfer film is peeled from the thermal transfer image-receiving sheet after printing, and thus, an image is formed on the thermal transfer image-receiving sheet.
Here, The demand for various multicolor printing matter is increasing. Thus, it is required to provide a composite thermal transfer sheet for multicolor printing, which has a colored thermal transfer image-receiving sheet. By using the aforementioned composite thermal transfer sheet for multicolor printing, the contrast between the heat-fusible ink and the colored thermal transfer image-receiving sheet can be improved, and the visibility of the printing matter is improved, so that the attractive printing matter is produced.
However, in this case, If a colored sheet such as colored plain paper on the market would be used as the colored thermal transfer image-receiving sheet of the composite thermal transfer sheet for multicolor printing, the concentration of a letter printed on the colored sheet is not uniform, and the unevenness of transfer is occurred, so that the printing quality is not stable. Further, there are not many kinds of color of the colored sheet, so that it is difficult to obtain the color sheet which has a desired or favorite color.
Furthermore, there is a problem of the background pollution, which the ink included in the thermal transfer film is removed and adhered onto a blank portion of the thermal transfer image-receiving sheet, when composite thermal transfer sheet is preserved for a long period, or preserved in a condition of high temperature. If the colored thermal transfer image-receiving sheet is used, since the contrast between the heat-fusible ink and the thermal transfer image-receiving sheet is improved, the background pollution is more noticeable in the colored thermal transfer image-receiving sheet. Therefore, a preservation environment and a preservation period are considerably restricted.